In a plain-paper copying (PPC) machine toner images applied to the surface of paper or other recording medium are fixated by application of heat and pressure. In certain PPC machines fixation is accomplished by passing the image-bearing recording medium between a hot thermal-fixation roll and a pressure roll. When this type of thermal-fixation device is used the toner material is directly contacted by a roll surface and a portion of the toner adheres to the roll surface. With subsequent rotation of the roll the adhered toner material may be redeposited on the recording medium resulting in undesirable offset images, stains, or smears; or, in severe cases, the recording medium may stick to the adhered toner material on the roll and become wrapped around the roll.
To counter these problems materials having good release properties such as silicone rubber or polytetrafluoroethylene are often used for the roll surfaces. Although improving performance of the thermal fixation devices, use of silicone rubber or polytetrafluoroethylene roll surfaces alone do not eliminate the problems. Another approach used to counter the problems is to include release agents with the toner materials to prevent them from adhering to the roll surface. These oilless toners also improve performance of the thermal-fixation devices but again, particularly in the case of high-speed type copying machines, do not completely eliminate the problems associated with toner pickup and transfer.
Toner pickup by the rolls can be controlled by coating the surface of at least one of the rolls of a thermal fixation device with a liquid release agent, such as a silicone oil. It is important that the release liquid be applied uniformly and in precise quantities to the surface of the roll. Too little liquid, or non-uniform surface coverage, will not prevent the toner from being picked up and redeposited on the roll. On the other hand, excessive quantities of the release liquid may cause silicone rubber roll surfaces to swell and wrinkle, thus producing copies of unacceptable quality. Furthermore, procedures intended to accommodate excess liquids by wiping or scraping them from the roll surface do not always produce favorable results and, in some cases, the efforts result in static electricity that causes further problems.
Devices to uniformly meter and coat a release liquid on copy machine roll surfaces are described in Japanese Laid-Open Patent Application Publication No. 62-178992. These devices consist of an oil permeation control layer adhered to a thick porous material which serves as a wick or reservoir for supplying oil to the permeation control layer. The permeation control layer is typically a porous polytetrafluoroethylene film which has been impregnated with a mixture of silicone oil and silicone rubber followed by a heat treatment to crosslink the silicone rubber. The thick porous material to which the permeation control layer is adhered is typically porous polytetrafluoroethylene tubing or felts of Nomex (TM) fibers, glass fibers, carbon fibers, or polytetrafluoro-ethylene fibers.
The devices described in Japanese Laid-Open Patent Application Publication No. 62-178992 meter and uniformly coat roll surfaces with release liquids at rates of 0.3 to 1.0 microliters/A4 size paper copy. They have been used successfully in copying machines and provide satisfactory performance until approximately 80,000 to 180,000 copies have been made. At this time, usually due to deformation and failure of the thick porous material supporting the permeation control layer or to separation of the permeation control layer from the thick porous layer, they can no longer perform acceptably and must be replaced.
In U.S. Pat. No. 5,232,499 to Kato et al. an improved device to meter and uniformly coat a surface with a release liquid is described. The device includes a liquid permeation control layer adhered to a porous support, the porous support also serving as a reservoir to supply a release liquid to the permeation control layer. Again, the permeation control layer is typically a porous polytetrafluoro-ethylene film which has been impregnated with a mixture of silicone oil and silicone rubber followed by a heat treatment to crosslink the silicone rubber. In this device, however, a rigid open-celled thermosetting polymer foam is used as a support material, and is filled with a mixture of silicone oil and silicone rubber which can reinforce the thermosetting polymer foam and provide oil to be supplied to the permeation control layer. This device meters and uniformly coats roll surfaces with release liquids at rates of 0.1 to 0.3 microliters/A4 size paper copy, and is capable of long-term operation in the temperature range of 150.degree. C. to 200.degree. C. They have been used successfully in copying machines, at an operating temperature range of 150.degree. C. to 200.degree. C., and provide satisfactory performance until more than 500,000 copies have been made. However, they lack the temperature resistance and durability required for use in high-speed copiers having operating temperatures in the range 200.degree. C. to 250.degree. C., and maintenance-free expectation of 700,000 to 1,000,000 copies. If used at operating temperatures of 200.degree. C. to 250.degree. C. premature failure due to deformation of the the thermosetting polymer foam, or separation of the permeation control layer from the support material occurs, and the device must be replaced with unacceptable frequency.
It is a purpose of this invention to provide a device for metering and coating a liquid on to a surface, for example, to the surface of toner image fixation rolls in a plain paper copying machine, at an operating temperature in the range 200.degree. C. to 250.degree. C., with exceptional accuracy, uniformity, and durability.